Production of magnesium nitride



' Nov. 15, 1949 w. DAvls PRODUCTION OF MAGNESIUM NITRIDE Filed April 1'7`, 1945 1w M065 ar Anna/WA maw/M l:

INVENTQR EW/f W @AV/f ,M ATTO R N EY5 Patented Nov. 15, 1949 PRODUCTION F MAGNESIUMI NITRIDE Lewis W. Davis, Beverly, Mass., assigner to Metal Hydrides Incorporated, Beverly, Mass., a corporation of Massachusettsk Application April 17, 1945, Serial No. 588,841

9 Claims. (Cl. 23-191) This invention relates to magnesium nitride and has for its object certain improvements in the method'of producing magnesium nitride.

It has been proposed to produce megnesium nitride by heating a body of magnesium metal in a. stream of nitrogen or ammonia. 'IAhis method has not achieved any commercial success, largely because the product produced is in fact not magnesium nitride but is a core of magnesium metal with a-coating of magnesium nitride. When, for example, magnesium powder or granules or liquid is treated with nitrogen gas in the customary manner, the nitriding reaction takes place only at the surface; so that the theoretical composition ofl magnesium nitride i-s not obtained. In

other words, what is obtained is partially-nitrided magnesium.

My investigations conrm my discovery that magnesium nitride of high purity, instead of a core of magnesium metal with a coating of magnesium nitride, may be produced, provided the method employed to nitride the magnesium is properly conducted.

In accordance with the invention, magnesium nitride is produced by reacting sublimed megnesium with nitrogen gas at a temperature below that at which the magnesium nitride dissociates but` at which the magnesiumvcombines with the nitrogen.

To this end a charge of magnesium metal is heated with applied heat to a temperature sufcientlyfhigh to initiate the nitriding reaction and to sublime or vaporize the magnesium. Magnesium tends to sublime below its fusion or melting point, which at normal pressures is 651 C., so that the temperature of the charge is advantageously kept below that temperature. The amount of sublimation is appreciable at temperatures as low as 550 C. A similar effect may, however, be obtained by reacting the vapors of magnesium with nitrogen gas. Since the boiling point of magnesium is 1110C at normal pressures, the charge must be considerably raised in temperature to vaporize the magnesium. Since sublimation of the magnesium begins well belowthe melting or fusion point and continues up to the boiling or vaporizing point of magnesium, the range of sublimation is rather wide, for example from about 550 C. to just below 1110 C. So far as the practice of the invention goes, it is unnecsary to boil the magnesium, although the temperature of the charge may be raised that high, if desired In any event, the temperature of the charge, and hence of the reaction zone in which the nitriding reaction is conducted, should be below the dissociation point of the magnesium nitride, which is 1500 C. at normal pressures.

Since the nitriding reaction is rstrongly exing upon operating conditions, it may be necesv sary to take special precautions to vavoid overheating the charge.

The charge is preferably in the form of relatively small pieces of magnesium metal, such as A granules, although other forms may, of course,

be employed. The pieces of magnesium are grad-v ually heated to a temperature sufficiently high to cause the nitriding reaction to begin. At this temperature the pieces of magnesium in touch with the nitrogen gas nitride at their surfaces, thus providing a coating or film of magnesium nitride on the pieces of magnesium. While this coating of magnesium nitride forms a barrier to further penetration of the nitrogen gas toward the interior of the pieces of magnesium, it prevents the pieces from sticking to one another or fusing into a solid mass as the temperature of the charge continues to rise.

Instead of stopping at this point, the heating of the partially-nitrided charge of magnesium is continued to effect the sublimation or vaporization of the un-nitrided magnesium in the interior of the coated pieces of magnesium. The sublimed or vaporized magnesium is forced through the coating or lm of magnesium nitride at the outside surfaces of the pieces of magnesium and then rises in the reaction zone in contact with the nitrogen gas. Each molecule of sublimed or vaporized magnesium is thus brought into reactive contact with nitrogen and is converted to magnesium nitride. The heating of the original charge of magnesium is continued until all, or

" substantially all, .of the metallic magnesium in the interior of the partially ntrided particles of magnesium, is thus expelled and nitrided.

In a presently preferred practice, a suitable temperature variation is maintained in the revtom thereof. The highest temperature in the reaction zone is maintained lat or near the bottom ofthe retort, while the upper part of the retort is kept relatively cool. As the sublimed or vaporized magnesium rises upwardly in the re- In that case, theY the retort is, of course, also magnesium nitride.

rThe retort is cooled and opened, after which the magnesium nitride is suitably removed.

To assure high purity magnesiumy nitride, it isadvantageous to confine the charge of magnesium in the retort and to evacuate the reaction zone and charge to remove air and moisture therefrom.v

Gradually heating the charge to a vtemperature short of that at which practical sublimationtakes place facilitates removal of the air and moisture` during the evacuating step. The retort is then filled with a suitable inert gas, such. as helium or argon. `Enough inert gas is preferably admitted to the retortto place the reaction zone andr charge under substantial positive pressure, for example pounds. When, therefore, nitrogen gas or a sourceof nitrogen gas', such asY ammonia, is admitted to the reaction Zone, and is consumed by the magnesium,the presence of the inert gas under positive pressure prevents the creation of a vacuum in the retort. While the nitriding reaction may be conducted under vacuum, it is disadvantageous in case' the retort should spring a leak. In thatcase, outside air and moisturewould seep into the retort until` the vacuum is broken. It is therefore better practice to maintain the reactionY zone in the retort under positive pressure withthe inert gas', because, in the case of a leak, inert gaslwould' seep out of the retort instead ofA air seeping into the retort.

In addition, the presence' of the inert gas in the retort facilitates temperature control of the nitriding reaction. It serves to dilute the nitrogen gas or ammonia admitted to the reaction zone, thus` helping to slowdown the nitrding reaction while avoidingY the danger of a vacuum being established in case the. retort should spring a leak.

While nitrogen gas may be admitted to the reaction zone, ammonia may bei similarly admitted as the source of' nitrogen gas. At the nitriding reaction, the ammonia is dissociated intoV nitrogen gas andv hydrogen gas, both gases being in nascent form and therefore highly reactive. The hydrogen gas tends to combine with surface oxide on the pieces of magnesium, to form moisture and then steam, and thus cleans or conditions the surfaces for nitriding with the' nascentV nitrogen. The nascent nitrogen tends also to react more readily with the sublimed or vaporized magnesium than does ordinary nitrogen gas.

"These and other features of the invention will be better understood by referring to the accompanying drawing, taken in conjunction with the following description, which illustrates diagrammatically an elevational View, mostly in cross-` section, of an apparatus that maybe employed in the practice ofthe invention.

The apparatus shovvn comprises an outer retort.

IIl supported Vwithinra heating furnace II, the retort being held in position by means of a plurality of supports I'2 spaced under and around a circumferential ange of ther retort and resting on the top of the heating furnace. The retort is made preferably of heat-resistant steel. It is provided with a removable cover I3 having attached thereto a vertical pipe I4 With valved gases: into. the1 chamber.

air and moisture from the' retorts.

' ture.

branches I5, IS and I1 connectible with a source of vacuum, nitrogen gas or ammonia, and inert gas, such as helium or argon, respectively. A gasket I8 is disposed between the cover and the flanged top portion of the outer retort, the three elements forming a non-leakable connection by means of a plurality of spaced bolts I 9. A removable inner retort 2-0 having. an. open top rests within the outer retort.

The heating furnace consists essentially of a box-like chamber 2| having a refractory bottom 22'side' and endI walls 23 and a top 24 with an opening of a size adapted to receive the outer retort. An expanding opening 25 is provided ator'near a lower corner of one of the side Walls of the'chamber, for the introduction of heating A ue opening 26 extends through another wall, preferably at a higher l`evel, so that heating gases passed into the chamber through opening 25 tend to pass around and in contact with the outer retort before leaving the chamber through the flue opening.

Theapparatus may be used as follows:v

A charge of magnesium metalfpreferably in smallor granular pieces, is placed' on the bottom of inner retort 20. The retort is then placed in outer retort I0; gasket I8 is placed in` position on the circumferential flange of the outer retort; cover I 3 is placed thereon;y and it is non-leakably' secured to the outer-retort by bolts ISJ In a presently-preferred practice, heating gases are passed through opening 25'- intoA heating chamber 2I, while the valve-in branch l5v isopened toevacuate The heating gases circulate around' and in contact with outer retort I0, Where they give up a good deal 0f their heat, after which the spent gases pass through flue opening 2'6. Heating ofthe charge in this manner is continued asits temperature rises and reaches' a point short of that at which practical subliminationof the magnesium occurs in order to facilitate removal of the air and: moisture. The valve'in branch I`5 is' closed when the evacuating stepis completed.

The valve in. branch I'I is then opened, in a. preferred? practice, to admit inert gas, such asv helium or argon, or both, to the retorts to replace the air evacuated therefrom. A sufficient amount of the inert-gas isadvantageously passed into the retort to break the vacuum and to place' and maintainthe reaction zone and charge under substantial' positive pressure, for example about 5 pounds. After the desired amount of inert gas' ha-s been admitted to the retorts, the Valve in branch I1 isiclosed,` or, if apressure valve is used, it may remainopen tol assure aconstant pressure of inert gas in the retorts.

The valve in branch I6 is then opened to admit a limited amount of nitrogen gas or ammonia,

or both, to the'retorts as the passage of heating gases throughv chamber 2| is continued, the heating gasesv being suciently hot gradually to heat the charge of magnesium to its nitriding tempera- When this-occurs, the surfaces of the pieces of magnesium in contact with' the nitrogen gas ,are promptly nitrided to form a coating, lm or layer of magnesium nitride thereon. The partially nitrided pieces of magnesium are in addition heated' to a temperature sufciently high to distill or'vaporizev theV 11n-nitrided magnesium in the interior portions of the pieces, so that the. sublimedV or vaporized magnesium is expelled" therefrom. 'As the sublimed or vaporizedk magnesium tends to ri'se in the "reaction zone of inner retort-20, it comes in contact with nitrogen gas and promptly reacts therewith to form additional magnesium nitride. This additional magnesium nitride isv in extremely finely divided form and tends to'deposit or condense along the inner Wall.

of the inner retort, as shown. To facilitate deposition of the magnesium nitride, the inner and outer retorts are provided with a suitable temperature gradient, the hottest portion being at or near the bottom thereof While the cooler portions are at the upper end. This desirable condition is obtained by having the upper portions of the inner and outer retorts extend a suitable distance above heating furnace Il into the open atmosphere so that heat may be rapidly dissipated therefrom.

As the nitriding action continues, regulated amounts of nitrogen gas or ammonia are admitted to the retorts. Due to the exothermic nature of the nitriding reaction, the amounts thus admitted are regulated in such a manner as to generate heat in amount to maintain the reaction zone at the nitriding temperature but below that at which the magnesium nitride dissociates. Control of the amount of heat generated is also effected in part preferably by regulating the amount of heat dissipated from the reaction zone. This control may be obtained, for example, by using retorts with a predetermined and optimum amount of radiating surface .and by placing a charge of magnesium of predetermined and optimum amount in the inner retort. Othermeans, such as cooling, may be employed to regulate the rate at which heat is dissipated from the retorts. After the nitriding reaction is given a good start initially, the use of .applied heat may be stopped; that is, passage of heating gases into chamber 2l may be discontinued. The heat necessary to complete the nitriding operation is then supplied solely by the generated heat. If the temperature of the reaction zone should fall too W, applied heat would, of course, -again be used.

In the construction shown, the top portion of inner retort is almost on a level with the upper portion of outer retort Ill. This arrangement tends to keep the magnesium nitride within inner retort 20. That is to say, the magnesium nitride formed from the sublimed or vaporized magnesium is condensed or deposited on the inner Wall of the inner retort before it has a chance to rise above the top portion of the inner retort.

As pointed out above, if ammonia is used as the source of hydrogen gas, on admission to the retorts, it is heated to a temperature sufficiently high to dissociate it into nascent nitrogen gas and nascent hydrogen gas. The nascent hydrogen tends to react with surface magnesium oxide, if present, on the pieces of magnesium metal in the ch-arge, to form moisture which, at the elevated temperatures employed, is promptly converted into steam. The nascent nitrogen then reacts with the cleansed metallic surfaces on the pieces of magnesium metal to form a coating, nlmV or layer of magnesium nitride. The nascent nitrogen appears to be more highly reactive than ordinary nitrogen gas as a nitriding agent.

When all of the metallic magnesium in the interior portions of the partially nitrided pieces of magnesium in the charge has been sublimed or volatilized, yand the nitriding reaction has gone to completion, the retorts and their contents are permitted to cool. Cover I3 is then removed from outer retort lll, inner retort 20 is removed from the outer vretort I0, and the magnum nitride is scraped or otherwise .removed from thek inner;

retort.

The operation described results-in asubstan-*I tially complete conversion of kall of the availabley magnesium inthe charge to magnesium nitride,Y

the efficiency of thenitriding reaction being exceedingly high, so that a product of high purity is obtained. The purer the magnesium placed in the inner retort, the purer will be the magnesium nitride produced by the nitriding operation.

Since the metallic magnesium in the interior portions of the pieces of magnesium metal is expelled into an atmosphere of nitrogen, complete nitriding of all of the magnesium metal is assured.

It will be clear that the above example is only by Way of illustration, and that various modifications thereof may be employed in the practice 'of the invention. For convenience, the magnesium With the pieces of magnesium at their surfaces toV form a coating of magnesium nitride thereon, gradually heating the partially nitrided charge to a temperature sufliciently high to sublime magnesium from the interior portions of the coated` pieces of magnesium and to make the sublimed magnesium break through the surface coating of magnesium nitride on the pieces of magnesium, gradually admitting additional amounts of nitrogen to the reaction zone, and converting the sublimed magnesium as it rises in the reaction zone to magnesium nitride with the nitrogen.

2. In the method of producing magnesium nitride, the improvement which comprises conning a charge consisting of a plurality of pieces of magnesium metal in a reaction zone, evacuating the reaction zone to remove air and moisture therefrom, admitting a limited amount of nitrogen to the reaction zone, gradually heating the charge to a temperature suiciently high to cause the nitrogen to react with the pieces of magnesium at their surfaces to form a coating of magnesium nitride thereon, gradually heating the partially nitrided charge to a temperature sufciently high to sublime magnesium from the interior portions of the coated pieces of magnesium and to make the sublimed magnesium break through the surface coating of magnesium nitride on the pieces of magnesium, gradually admitting additional amounts of nitrogen to the reaction zone, and converting the sublimed vmagnesium as it rises in thereaction Zone to magnesium nitride with the nitrogen.

3. In the method of producing magnesium ni' tride, the improvement which comprises conning a charge consisting of a plurality of pieces of magnesium metal in a reaction zone, evacuating' the reaction zone to remove air and moisture therefrom, admitting inert gas to the reaction sure, admitting a limited amount of nitrogen to the reaction zone, gradually heating the charge'A magere.;

toV a temperature sufficiently high to cause the nitrogen to react with the pieces of magnesium atltheir surfaces to form a coating. of magnesium nitridel thereon,N graduallyl heating the partially nitrided charge to a temperature sumcientiy high to sublime magnesium from the interior portions of` theY coated pieces of magnesium and to make the sublimed magnesium break through the surfac'ei coating of magnesium nitride on the pieces of magnesium, gradually admitting additional amounts lof nitrogen to the reaction zone, and

converting theA sublimed magnesium as it rises in` the reaction Zone to magnesium nitride with the nitrogen.

4. In the method of producing magnesium nitride; the improvement whichl comprises conning a charge consisting of a plurality of pieces of magnesium metal in a reaction zone, evacuating the reaction zone to remove `air and moisture` therefrom, admitting a limited-amount of nitrogento the reaction zone, gradually heating the charge to a temperature sufliciently high to cause the nitrogen to react with the pieces of magnesium at their surfaces to form a coating of magnesium nitride thereon, gradually heating the partially nitrided charge to a temperature sumciently high to sublime magnesium from the interior portions of the coated pieces of magnesium and to make the sublimed magnesium break 5. In the method of producing magnesium nitride, the limprovement which comprises conning a charge consisting of a plurality of pieces of magnesium metal in a reaction zone, admitting a limited amount of nitrogen to the reaction zone, gradually heating the charge with applied heat to a `temperature suliciently high to cause the nitrogen to react with the pieces of magnesium at their surfaces to form a coating of magnesium nitride thereon, gradually heating the partially nitrided charge to a temperature sufficiently high to sublime magnesium from the interior portions of the coated pieces of magnesium and to make the sublimed magnesium break through the surface coating of magnesium nitride on the pieces of magnesium, terminating the use of applied heat, gradually admitting additional amounts of nitrogen to the reaction zone, converting the sublimed magnesium as it rises in the reaction zone to magnesium nitride with the nitrogen, and regulating the rate of nitrogen ythus admitted to the reaction Zone to an amount Which on combining with the sublimed magnesium generates heat in` sucient in amount to decompose the magnesium nitride but lsufcient in amount to carry the nitriding Operation to completion.

6. In the method of producing `magnesium nitride, the improvement which comprises confining a charge consisting of a plurali-ty of pieces of magnesium metal in a reaction zone, evacuating the reaction zone to remove air and moisture therefrom, admitting inert .gas to the reaction zone in amount sucie'nt torplace and maintain it and the charge under substantial positive pressure, admitting a limited amount of nitrogen to the reactionl zone, gradually heating the charge g with applied heat to a temperature suiiiciently highto cause the nitrogen to react with the pieces oi magnesiumat their surfaces to form a coating of magnesium. nitride thereon, gradually heating the partially nitrided charge to a temperature. sumciently high to sublime magnesium from the interior portions of the coated pieces of magnesium and to make the sublimed magnesium break through the surface coating of magnesium nitride on the pieces of magnesium, terminating the use of applied heat, gradually admitting additional amounts of nitrogen to the reaction zone, converting the sublimed magnesium as it rises in the'reaction Zone to magnesium nitride With the nitrogen, and regulating the rate of nitrogen thus admitted to the reaction Zone to an amount which on combining with the sublimed magnesium generates heat insufficient in amount to decompose the magnesium nitride but suiiicient in amount to carry the nitriding operation to completion.

7. In the method of producing magnesium nitride, the improvement which comprises placing a charge of magnesium in a conned reaction Zone, evacuating the reaction zone to remove air and moisture therefrom, heating the reaction zone to maintain the magnesium above its sublimation temperature but below its boiling temperature, admitting a limited amount of a gas selected from the group consisting of nitrogen and ammonia to the reaction zone to initiate the nitriding reaction and limit the exothermic heat generated by the reaction to an amount insufflcient to raise the temperature of the reaction zone above said -boiling temperature, reacting nitrogen with the charge of magnesium at its surface to form a coating of magnesium nitride thereon, heating the partially nitrided charge to a temperature sulciently high to cause sublimed magnesium to break through the surface coating of magnesium nitride on the charge and to cause the sublimed magnesium as it moves away from the charge to react With nitrogen to form magnesium nitride, gradually admitting further amounts of said gas to the reaction zone to con' tinue the nitriding reaction, and regulating the rate -of formation of the magnesium nitride and hence the rate of generation of exothermic heat bycontrolling the rate at which said further amounts of said gas are admitted and thereby maintain the temperature of the reaction zone below said -boiling temperature.

8. In the method of producing magnesium nitride, the improvement which comprises confining la charge of magnesium metal in a reaction zone, `admitting a limited amount of nitrogen to the reaction Zone, gradually heating the charge to -a temperature suiciently high to cause the nitrogen to Ireact with the charge of magnesium at its surface to form a coating of magnesiumnitride thereon, gradually-heating the partially nitridedcharge to a temperature sufficiently high to sublime magnesium from the interior portion of the lcoated charge of magnesium and to make the sublimed magnesium break through the surface coating of magnesium nitride on the charge of magnesium, Ygradually admitting additional `amounts of nitrogen to the reaction zone, and

converting the sublimed magnesium as it rises in the reaction zone vto magnesium nitride with the nitrogen.

' 9. In the method -of producing magnesium nitride, the improvement which comprises conlining a charge of magnesium metal in a reaction zone, evacuating the reaction Zone to remove air and moisture therefrom, admitting inert gas to the reaction zone in amount suiicient to place and maintain it and the charge under substantial positive pressure, admitting a limited amount of nitrogen to the reaction zone, gradually heating the charge to a temperature suiciently high to cause the nitrogen to react With the charge of magnesium at its surface to form a coating of REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,319,843 Burke Oct. 28, 1919 1,803,720 Miner May 5, 1931 OTHER REFERENCES Maxted, Ammonia And The Nitrides, 1921 ed., J. & A. Churchill, London, publishers, pages 49, 54-56 and 67-68.

Mellors Modern Inorganic Chemistry, 1939 ed., Longmans, Green & Co., N. Y., publishers, pages 385 and 386.

Mellor, A Comprehensive Treatise on Inorganic and Theoretical Chemistry, 1928 ed., vol. VIII, Longmans, Green & Co., N. Y., publishers, page 104.

Certificate of Correction Patent No. 2,488,054 November 15, 1949 LEWIS W. DAVIS It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 1, line 4, and lines 25 and 26, for megnesium read magnesium; column 4, line 42, for sublimination read sublimation; column 10, list of references cited, line 16, for pages 385 and 386 read pages 622 and 623;

and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Oice.

Signed and sealed this 7th day of March, A. D. 1950.

THOMAS F. MURPHY,

Assistant Commissioner of Patents. 

